Technology: Kubernetes is the open source cluster management tool designed by Google, and donated to cloud native computing foundation. It mainly used to automate the deployment process, scaling of the application, operations on application hosted containers across the cluster nodes. It usually works with Docker container, co-ordinates with wide range of Docker running cluster hosts.
- Cluster: A cluster is a set of virtual machines used by kubernetes to deploy/run our applications.
- Pod: A pod is set of one or more containers (Docker containers) that share the storage and holds the options to how to run the containers. Pods are co-located and co-scheduled and run in the shared context.
- Node: A Node is worker machine in kubernetes. It may be VM or a physical machine depending on the cluster. Each node must contain list of services like Docker, Kubelet and kube-proxy to run pods and managed by master components.
- Label: Labels are key/value pairs attached to objects such as pods, which are used to identify the attributes of objects meaningful and relevant to users, but these will not directly imply core semantics. Labels can be used to organize and to select subsets of objects. These can be used to attach to objects at creation time, later can be modified at any time.
- Selector: A selector is an expression that matches label to identify related resources such as which pods are targeted by a load balancer.
- Replication Controller: A ReplicationController verifies that a defined number of pod “replicas” are operating at any time. It makes sure that a pod or similar set of pods are always up and available. If there are many pods available, it will kill some. If there are too less, the ReplicationController will start more. Unlike manually developed pods, the pods maintained by a ReplicationController are automatically replaced if they fail, get deleted, or are terminated.
- Service: A service is middle layer between front-end and back end, service is an abstraction which defines a set logical pods, and policy by which to access them. Service will target a set of pods using label selector.
Volume: Volume is directory on file system with some data in it, which is accessible to containers. As Kubernetes volumes are built on top of Docker Container volumes, and providing the additional features compared to Docker volumes.
- Secrets: A Secret is an object that store the sensitive information like passwords, authentication tokens etc… this information can be put in either in image file or pod specific volume. A secret can be used at pod in two ways:
- As files mounted one or more container volume.
- Used by kubelet pulling image.
- Name: All objects in the Kubernetes REST API are unambiguously identified by a Name and a UID. UID`s are generated by kubernetes internally.
- Name Space: Name spaces will be used in environment with many users, multiple teams involved. If you are familiar with spring framework, it provides one separate name space for each of its module.
- Annotation: A key/value pair that holds arbitrary non-identifying metadata for retrieval API by tools, libraries etc… This information may be human readable format, it may be structured, or unstructured, and it may include characters which are not allowed in labels. Efficient filtering by annotation value is not supported.
Kubernetes and Java development in India on top of Docker, so we need to install Docker to run Kubernetes.
- Windows: we can download InstallDocker.msi (https://download.docker.com/win/stable/InstallDocker.msi) and we can install msi file, by default it will create VM. We can run Docker Quick Start Terminal. We can verify the docker installation by running docker –version command.
- Ubuntu: we can install Docker in two ways:
- We can download deb package, install it manually and we need to manage upgrades manually (sudo dpkg -i docker-engine.deb).
- We can setup Docker repository and we can install from added repository.
- Repository setup:
- Execute the below commands in terminal to add the repository.
- sudo apt-get install apt-transport-https ca-certificates
- curl -fsSL https://yum.dockerproject.org/gpg | sudo apt-key add –
- apt-key fingerprint 58118E89F3A912897C070ADBF76221572C52609D
- sudo add-apt-repository “deb https://apt.dockerproject.org/repo/ ubuntu-$(lsb_release -cs) main”
- Execute the below commands to install docker:
- sudo apt-get update
- sudo apt-get -y install docker-engine
- By default it will always install latest version from repository, if we want to install specific version then we can execute the below command instead of above command.
- To get the list of versions in repository we can run the below command:
- apt-cache madison docker-engine
- Repository setup:
Minikube is the tool, using which kubernetes will run locally, and also it will setup the single node cluster in our dev machines.
Minikube supports Kubernetes features such as:
- ConfigMaps and Secrets
- Container Runtime: Docker, and rkt
- Enabling CNI (Container Network Interface).
Installing MiniKube in windows:
Download the minikube from https://storage.googleapis.com/minikube-builds/101/minikube-windows-amd64.exe
And go to saved folder and invoke command in command prompt to install minikukbe.
And we need to download kubectl.exe from https://storage.googleapis.com/kubernetes-release//v0.18.0/bin/windows/amd64/kubectl.exe
And add to path (environment variable).
Creating Docker image for our application:
Build tools like Maven, Gradle provides plugin for creating docker image.
If we are using maven as build tool then we need to add the below docker plugin:
dockerDirectory refers to folder where docker file present.
directory: refers to build directory.
We can run mvn clean package docker: build to create docker image.
If we are using continuous integration tools like Jenkins we need to push image after the build successful, we can push the image using docker:push goal.
We can also tag the image using imageTags in configuration.
Pushing Docker images to private repository:
We can use registry docker image to start the private repository.
docker run -d -p 5000:5000 –restart=always –name registry registry:2
and change image name specified in docker maven plugin to “<imageName>localhost:5000/hello-world</imageName>”.
And run the mvn clean package docker:build it will build the image file.
Deploying docker image into Kubernetes:
We need to push the above docker image into registry, and we need to execute the below command:
kubectl run hello-world –image=localhost:5000/hello-world:v1 –port=8080
It will deploy our docker image into kubectl cluster, we can also configure under which port our application want to run using port parameter.
We can run the kubectl get deployments to know the deployment status of our application.
View our app: by default all our application will also accessible in kubectl cluster.
For accessing our application we can do in two ways:
- We can create kubectl proxy between terminal and kubectl cluster.
For this we need to run kubctl proxy command to create proxy.
- We can expose our application using kubectl expose command.
kubectl expose deployment/hello-world –type=”NodePort” –port 8080
We can run kubectl services to check the service is exposed to public or not.
To get the details about our application service we can run the below command:
kubectl describe services/hello-world
Describe output will provide nodePort of the service, we need to use this port for accessing our application.
Updating Docker Image:
We can build latest docker image and we can update deployment path for application to update the application in kubernates.
kubectl set image deployments/hello-world hello-world=localhost:5000/hello-world:v2
Conclusion: Kubernetes is the tool for creating cluster for docker like containers, we can easily scale the cluster size, and we can easily deploy our docker image in all of the configured cluster using kubernets easily.